Equivalence of pure propane and propane TE gases for microdosimetric measurements

A tissue-equivalent proportional counter (TEPC) simulates micrometric volumes of tissue, if the energy deposited in the counter cavity is the same as that in the tissue volume. Nevertheless, a TEPC measures only the ionizations created in the gas, which are later converted into imparted energy. Therefore, the equivalence of the simulated diameter in two gases should be based on the equality of the mean number of ions pairs in the gas rather than on the imparted energy. Propane-based tissue equivalent gas is the most commonly used gas mixture at present but it has the drawback that its composition may change with time. From this point of view, the use of pure propane offers practical advantages: higher gas gain and longer stability. In this work, microdosimetric measurements performed with pure propane, at site sizes between 0.05 mg/cm2 and 0.3 mg/cm2, demonstrate that the response of a propane-filled detector in gamma and in neutron fields is almost the same if an appropriate gas density is used.